The currently available extension cables for image transmission include VGA (Video Graphics Array) extension cable, DVI (Digital Visual Interface) extension cable, and HDMI (High-Definition Multimedia Interface) extension cable. The VGA cable is used to transmit only analog signals, while the DVI cable can be used to transmit both analog and digital signals. However, both the VGA and the DVI cable could not be used to transmit audio signals. On the other hand, the HDMI cable can be used to transmit digital signals, audio signals and different network data.
A VGA extender, which is commercially available in the market, is often used in long-distance analog signal transmission. However, even with the VGA extender, signal delay or signal attenuation would still occur if the transmission cable between an image output source and an image display device is excessively long. As a result, in the case of a long-range transmission over 40 meters, the VGA image signal played back at a remote location is subjected to the following distortion conditions: (1) reduced image brightness, (2) blurry image, and (3) unsynchronized RGB colors.
Please refer to FIG. 1, which is a conceptual diagram of a conventional long-range VGA image transmission system requiring manual image adjustment. As shown, an image-enhancement compensation circuit 22 must be provided between an image output source 20, such as a computer, and an image display device 21, such as a screen, so that an output image signal transmitted via an image encoding and transmitting circuit 23 to an image decoding and receiving circuit 24 can be adjusted via the image-enhancement compensation circuit 22 to correct any distortion thereof. An operator has to observe the image display device 21 with the naked eye to check the distortion conditions and makes adjustment and compensation of different degrees according to different distortion conditions, so that the image after compensation can have an image quality closer to that of the original image signal from the image output source 20.
A disadvantage in using the image-enhancement compensation circuit is that the image signal is manually adjusted. Thus, extra manpower is required to do image adjustment during the electromechanical work in general building construction. This would cause difficulties in installing the transmission system and increased labor cost. Further, in installing the image display system in a working site, the operator has to determine the values for five different image adjustment functions through observation with the naked eye. Therefore, an experienced operator is needed to perform such image adjustments, so that the image can be accurately adjusted to have a quality close to that of the original image. Frequently, the image adjusted by an operator of insufficient experience would still have some degree of distortion when it is displayed on the image display device.
In view that the conventional long-range VGA image transmission system requiring manual image adjustment has the disadvantages of uneasy to make accurate adjustment and requiring extra labor cost, it is tried by the inventor to develop an improved long-range VGA image transmission system that provides automatic image adjustment to overcome the disadvantages in the prior art.